(1) Field of the Invention
The present invention relates to an image forming apparatus such as a digital copier, the printing unit of a facsimile machine, digital printer, plotter etc., and more particularly relates to an image forming apparatus in which an image is formed on a recording medium by causing the developer to jump thereto.
(2) Description of the Prior Art
There has conventionally been known an image forming technique by which electric fields are produced in apertures in accordance with an electric signal to control charged particles passing through the apertures, thus forming a visual image corresponding to the image signal, onto a recording medium such as paper etc.
For example, Japanese Patent Application Laid-Open Sho 58 No. 104,769 discloses an image forming apparatus wherein an image is directly formed on a recording medium by causing charged particles to jump and adhere onto the recording medium by electric force under the application of electric fields whilst varying the voltage which is applied to a control electrode having a plurality of passage holes and placed in the jumping path.
However, in this conventional art, since no consideration has been given to the virtual capacitance which arises at the control electrode when it has been mounted and packaged, this virtual capacitance causes a variety of adverse influences.
For example, if a control electrode has inherent capacitance and virtual capacitance, the combined capacitance of these two may degrade the quality of the image and break down its high withstand voltage driver. Alternatively, there is a risk that the consumed current determined by the capacitance and the voltage could affect a person's body.
Inherently, in an image forming apparatus of this type, which performs electric field control, the power consumption increases in proportion to the capacitance of the control electrode. Therefore, it is preferred that the virtual capacitance is made as low as possible to reduce the power consumption.
Since the control electrode of the image forming apparatus needs interconnections for connecting each of the control electrode elements to the driver source, a shield electrode which is placed opposite the interconnections of the control electrode to shield external noise and radiation noise. Further, the control electrode also needs a fixing means for supporting the control electrode. Accordingly, unexpected virtual capacitance will occur between the control electrode and the lines of the control electrode elements due to these interconnections and fixing means.
Therefore, it is a very critical problem as to how the virtual capacitance, which will change depending upon the material, shape, structure, etc. of the control electrode, is dealt with.